runtime(vim): Update syntax file (#13653)

Improve variable highlighting in :let, :unlet, :const and :for commands.

Match registers and local, global and terminal option variables.

Signed-off-by: Doug Kearns <dougkearns@gmail.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>

runtime/syntax/testdir/input/vim_variables.vim

@@ -1,7 +1,16 @@
 " Vim variable highlighting
 
+" :let
+
 let foo = expr
 
+let foo[0] = expr
+
+let foo[1:2] = expr
+let foo[:2] = expr
+let foo[1:] = expr
+let foo[:] = expr
+
 let foo += expr
 let foo -= expr
 let foo *= expr
@@ -9,3 +18,121 @@ let foo /= expr
 let foo %= expr
 let foo .= expr
 let foo ..= expr
+
+let $FOO = expr
+let $FOO .= expr
+
+let @f = expr
+let @f .= expr
+
+let &foo = expr
+
+let &t_k1 = "\<Esc>[234;"
+
+let &foo .= expr
+
+let &foo += expr
+let &foo -= expr
+
+let &l:foo = expr
+
+let &l:foo .= expr
+let &l:foo += expr
+let &l:foo -= expr
+
+let &g:foo = expr
+
+let &g:foo .= expr
+let &g:foo += expr
+let &g:foo -= expr
+
+let [foo, bar] = expr
+let [foo,
+      \ bar] = expr
+
+let [foo, bar] .= expr
+let [foo, bar] += expr
+let [foo, bar] -= expr
+
+let [foo, bar; baz] = expr
+let [foo,
+      \ bar;
+      \ baz] = expr
+
+let [foo, bar; baz] .= expr
+let [foo, bar; baz] += expr
+let [foo, bar; baz] -= expr
+
+let foo =<< END
+...
+END
+let foo =<< trim END
+...
+END
+let foo =<< eval END
+...
+END
+let foo =<< trim eval END
+...
+END
+let foo =<< eval trim END
+...
+END
+
+let foo
+let foo bar
+
+" :unlet
+
+unlet foo
+unlet foo bar
+unlet foo
+      \ bar
+
+unlet! foo
+unlet! foo bar
+unlet! foo
+      \ bar
+
+unlet $FOO
+unlet! $FOO
+
+unlet list[3]
+unlet list[3:]
+unlet dict['two']
+unlet dict.two
+
+" :const
+
+const foo = expr
+
+const [foo, bar] = expr
+
+const [foo, bar; baz] = expr
+
+const foo =<< END
+...
+END
+const foo =<< trim END
+...
+END
+const foo =<< eval END
+...
+END
+const foo =<< trim eval END
+...
+END
+const foo =<< eval trim END
+...
+END
+
+const foo
+const foo bar
+
+" :for
+
+for foo in expr
+endfor
+
+for [foo, bar] in expr
+endfor